Field of the Invention
The present invention relates to synthesis of ethanol and other higher alcohols from syngas. More particularly, it relates to supported molybdenum-containing catalyst compositions for accomplishing this synthesis.
Background of the Art
Alcohols, particularly those ranging from methanol to hexanol (C1-6OH), are important products that can be synthesized from synthesis gas. Synthesis gas (“syngas”) is a mixture of hydrogen gas and carbon monoxide gas (H2/CO). The produced alcohols are useful both as fuels and as chemicals for a variety of manufacturing processes. Among these, those defined herein as higher alcohols, i.e., alcohols having at least two carbon atoms (C2+), are currently sought to serve as, for example, automobile fuels and fuel blends. In these applications they may offer desirably high octane number along with desirably low emissions of nitrogen oxide (NOx), ozone, CO, and aromatic vapors. In addition, the higher alcohols may be useful as alternative feedstocks for important olefins (produced via dehydration of the alcohol), particularly when the syngas is derived from biomass or coal.
Useful higher alcohols may be directly and catalytically synthesized from syngas. Researchers have identified three general types of catalysts that are capable of carrying out this synthesis. These types include: (i) mixed metal oxides promoted with alkali metals, which are known as “modified methanol synthesis catalysts;” (ii) alkali-doped molybdenum sulfide catalysts; and (iii) noble and/or transition metal catalysts on oxide supports. Although such catalysts are often used with the intention of producing higher alcohols in particular, each of these three catalyst types tends to show preferential selectivity toward methanol (CH3OH, alternatively “MeOH”) and/or methane (CH4). Thus, yields of the higher alcohols in such processes are often poor.
Because of this preferential selectivity, much research has been devoted to improving the yield of the higher alcohols, particularly ethanol, propanol, and butanol (C2H5OH, alternatively “EtOH”; C3H7OH; and C4H9OH, respectively; i.e., C2-4OH). U.S. Pat. Nos. 4,590,314 and 4,609,678 disclose processes for making an alcohol mixture, preferably containing no more than 85 weight percent (wt %) methanol, wherein catalysts of copper/thorium/alkali metal oxides are used. U.S. Pat. Nos. 4,607,055 and 4,661,525 disclose catalysts consisting of molybdenum (Mo) and cobalt (Co), iron (Fe), or nickel (Ni), promoted by an alkali metal. In another example the catalyst is an alkali metal-promoted cobalt/molybdenum sulfide (Co/MoS2) that has been mixed with a clay or impregnated with nanotubes. U.S. Pat. No. 4,675,344 teaches using Mo or tungsten (W) catalysts to convert a syngas feed including hydrogen sulfide (H2S). However, where fewer than 57 parts per million by volume (ppmv) of H2S is included in the feed and a potassium carbonate/cobalt/molybdenum sulfide/carbon (K2CO3/Co/MoS2/C) catalyst is used, MeOH is the dominant product.
U.S. Pat. No. 7,449,425 discloses use of an ionic clay-supported rhenium/manganese (Re/Mn) catalyst. The product selectivity to MeOH is 65.4 wt %; EtOH is 4.8 wt %; dimethyl ether (DME) is 6.1 wt %; and methane is 19.9 wt %, respectively, when the catalyst is a combination of 3 wt % Re and 3 wt % hydrated manganese/magnesium aluminum dihydroxide carbonate (Mn/Mg0.34Al0.66(OH)2(CO3)0.33.mH2O), where m is the number of moles of the water of hydration and is an integer ranging from 0 to 2. U.S. Pat. No. 7,314,960 discloses the catalytic synthesis of an oxygenate from an alcohol over a copper/zinc/magnesium/aluminum (Cu/Zn/Mg/Al) based catalyst. U.S. Pat. Nos. 7,700,810; 7,700,811; 7,700,813; and 7,705,192 disclose conversion of C1-2 alcohols (MeOH and EtOH) to 1-butanol and isobutanol on a thermally decomposed Cu/Zn/Mg/Al hydrotalcite catalyst. U.S. Pat. No. 7,718,832 discloses a combination catalytic process for producing EtOH from syngas using a series of three different catalyst beds within a single reactor. The first stage is the hydrogenation of CO to form MeOH, and the other beds serve to promote both the homologation of MeOH with H2 and CO to form EtOH, and the hydrogenation of other oxygenates to form higher alcohols.
Despite the many efforts to identify catalysts enabling improved selectivity to higher alcohols (C2+OH), there remains a need for even greater selectivity enhancement. Greater control of selectivity, combined with the low cost and high availability of syngas, offers the possibility of lower cost and more convenient manufacture of a wide variety of products based on alcohols.